To stimulate the production, low-permeability rock (methane-containing coal beds, shales, dense gas-bearing sandstones) is often hydraulically fractured, using a small amount of proppant and sometimes even without using it. This opens naturally occurring fractures and microfractures in the reservoir or generates new fractures which may improve considerably the hydrodynamic connection between the reservoir and the wellbore. However, it is impossible to predict the fracture opening degree as there is a wide variety of influencing factors. Therefore, it is often impossible to select a proper type of proppant. As a result, most of fractures close after the fracturing pressure has been relieved. Moreover, proppant preparation, manufacturing and grading processes take a lot of time.
Intense injection of nitrogen into a reservoir (i.e. injection of pure nitrogen into very low-permeability rock) is a typical example of the proppant-free fracturing. The produced fracture is expected to maintain a sufficient degree of permeability for efficient production, taking into account low permeability of the reservoir. However, the wellbore/fracture network connection caused by stress concentration around the wellbore is still one of the main problems.
There is a common well productivity enhancement method according to which a slurry of a nonexplosive breaking agent that expands while hardening, is injected into a well as a fracturing fluid, at a hydration pressure exceeding the displacement pressure. The reservoir is then hydraulically fractured, the fracturing fluid is displaced with a displacement fluid until a near-wellbore fractured region free of fracturing fluid is formed, and the well is kept under displacement pressure until the fracturing fluid hardens in the fractures (RF Patent No. 2079644, 1997). The said method provides generation of additional fractures or additional opening of existing fractures. The produced fractures are not filled with a hard material but remain empty or are filled with a reservoir fluid, thus increasing the permeability of the near-wellbore region and enhancing the productivity of the well.
However, this method offers no solution to the problem that arises in the near-wall region where the stress which causes the fractures to close has the highest value and increases as the pressure decreases in the wellbore. The fracture mouth plugging hampers the optimization of oil production and is the main disadvantage of this method and of many other well-known techniques.